It’s very easy, I eat whatever I want… I have experience, I was a vegetarian and well, my diet is way, way more restrictive than back then I am lucky with my relatives and when it’s not the case, I ignore them very well.
Your case is simple: it’s posion to you. I guess it’s a bigger problem when one needs to fight with their own little tempting demons too…
No one even tried to force me to eat carbs, I think… Food in general, yes (and some of my relatives didn’t get my vegetarianism either). I said no when I didn’t want to eat (because I was full, eaten a TON already, I really could eat) and they yelled and I was pretty calm. It’s their problems, not mine. I am very good at being super selfish if it’s about MY BUSINESS. Even if I am not allergic, I put into my body whatever I want or have or something but other people’s desires have close to no effect on it. I don’t want, I don’t eat. My reason doesn’t matter, it’s my own decision and I have a right for it.
It’s way more problematic if the conflict is in someone’s own home especially if the one in question can’t get their own food… But I guess the chains inside (old habits, wrong believes, I want to please every, whatever) can be very bad as well. I have moderately little of them and if a food definitely harms me, it’s easy, that’s NOT food.
HOW to tell someone no or to shut up, it depends on our temper, relationship with the other person and so on. I have a temper, I am honest and I gladly argue for a little while especially if I see any point in it But if I had such an encounter often, I probably would just ignore it. Less effort. I never ever would lie despite it’s a very beloved method to avoid eating (carbs or food in general).